The present invention relates to a broach and method for preparing a medullary cavity for receiving a stem component of a femoral prosthesis, specifically an improved machining assembly and method for preparing a medullary cavity that allows for the implantation of a femoral prosthesis that is neutrally aligned within the prepared medullary cavity for improved fixation therein, thereby minimizing any subsequent loosening and pain commonly associated with mis-aligned prostheses.
It is widely known that the success of cementless hip replacement depends on correct sizing and placement of prosthesis within the medullary cavity of the femur. In cases that fail due to aseptic loosening, a common observation is that the implant is not large enough to achieve support through filling the implant site and thereby achieving contact with the cortical surfaces of the femur. Typically, these undersized prostheses are mis-aligned and have been placed in some degree of varus inclination with respect to the medullary axis of the femur. Consequently, the implant only achieves contact with the cortex of the femur medially, immediately below the surface of the proximal femoral osteotomy, and laterally, adjacent to the distal tip of the prosthetic stem. However, contact is not present in other areas, including the anterior and posterior cortices, the medial cortex beneath the lesser trochanter, or the lateral cortex above the distal tip of the implant. Contact in these areas is necessary to rigidly fix the prosthesis and to prevent excessive relative motion at the implant/bone interface which would lead, ultimately, to pain and loosening.
In practice, prostheses that are implanted in the femur are undersized because the opening formed by the surgeon does not extend far enough laterally within the proximal femur to allow the prosthesis to be aligned with the longitudinal axis of the canal. This occurs because the medullary axis passes through the superior surface of the femur in the vicinity of the medial edge of the greater trochanter, close to the posterior cortex of the femoral neck, in an area of strong bone which is difficult to machine using conventional instruments, including broaches, rasps, and reamers. As the bone adjacent to this area is relatively soft, conventional machining instruments tend to be deflected away from the hard bone and enter the medullary canal at a site that is located more anteriorly and medially. As subsequent instruments enlarge the initial point of entry, bone is progressively removed from the anterior and medial walls of the cavity, leading to the development of a mis-aligned implantation site.
If a broach or rasp is placed into a femoral canal through an entry hole that is not aligned with the medullary axis, the broaching or rasping instrument will become wedged in the canal because the teeth present on the devices are generally incapable of effectively removing areas of hard bone which block their advancement. Removal of this bone, generally, can only be effected by rotating machine tools, such as flexible or rigid reamers or, possibly, bone chisels. Thus, in conventional hip replacement, preparation of a neutrally-aligned implantation cavity depends critically upon the initial entry point of instruments into the femur and the use of adjunctive instruments to machine away the areas of hard bone that block the motion of the broach as it seeks to achieve neutral alignment within the canal.
The anatomic variability of the proximal femur also contributes to mis-alignment and undersizing of femoral prostheses. The medullary axis panes through the superior surface of the femur at a point that varies by +/-5 mm medial-laterally and anterior-posteriorly, the precise location depending upon the shape of the metaphysis and the orientation of the femoral neck with respect to the rest of the femur. For this reason, it is difficult to determine intraoperatively whether instruments placed in the femoral canal are correctly aligned and whether the entry point for instruments designed to machine the femur is situated sufficiently laterally and posteriorly to allow development of a neutrally aligned implantation site.
The conventional approach to minimizing the difficulties in preparing the femur in the presence of areas of hard bone has been to insert a reaming instrument, typically a long, conical reamer or medullary drill, into the medullary cavity and to assume that the instrument would be co-axial with the medullary cavity during its insertion, in effect forcing the proximal part of the instrument to cut into the greater trochanter to an extent necessary to provide correct alignment of the prepared cavity. In practice, this approach has not been entirely successful because of the following factors:
(1) The bone within the trochanter is generally very hard, greasy, and often covered with a considerable quantity of soft tissue which tend to impair the cutting action of conventional drilling and reaming instruments. PA1 (2) This approach assumes that the instrument will be aligned with the medullary cavity without enlarging or cutting the bone distally, thereby distorting the cavity itself. In practice, the use of long drills and instruments that are not smooth-tipped can lead to mis-alignment because the instrument cuts bone both proximally and distally. PA1 (3) The entire approach of using the intramedullary cavity to guide the alignment of instruments assumes that the diaphyseal and metaphyseal segments of the femur coincide. In practice, however, there is a deviation of up to two degrees between the axis of the metaphysis and the diaphysis of the medullary cavity. Consequently, the path machined by the distally-guided instrument only gives a rough indication of where a neutrally-positioned instrument should lie within the metaphysis. PA1 (4) Aggressive reaming instruments and drills often remove a considerable segment of the greater trochanter during their insertion. This necessitates greater exposure of the surgical site and removal of excessive bone, and may lead to increased long-term osteolysis as soft cancellous bone is exposed to wear debris generated within the joint.
There exist many devices that assist the process of shaping the femoral canal to match the contours of the femoral implant; however, the successful function of each of these devices is predicated on the assumption that some instrument has been inserted into the medullary canal in a neutral position. Such an instrument is then used as a platform for locating reamer guides of a variety of designs. Such instruments have been available commercially and are primarily used to facilitate reaming of the medial bone within the femur, which is often very strong and may prevent the use of a correctly sized prosthesis. Exemplary instruments are manufactured by Biomet, Inc. (Warsaw, Ind.) and are also disclosed in U.S. Pat. No. 4,809,689 (Anapliotis), U.S. Pat. No. 4,777,942 (Frey, et al.), and U.S. Pat. No. 4,738,256 (Freeman et al.).
Additional instrument designs are available for use with modular prostheses comprising interchangeable anterior and posterior elements which are mounted on the central stem of the prosthesis. The Richards Medical Company (Memphis, Tenn.) manufactures a system consisting of a plurality of modular reaming guides that utilize a rail attached to a stent placed in the medullary canal. The function of the rail is to guide the position of the reamer used to machine both the anterior and posterior surfaces of the femur to optimize the fit of this prosthesis within the femur.
Another design is disclosed in U.S. Pat. No. 5,169,402 (Elloy) and comprises a broach having two separate parts that connect on a articulating handle to form a solid, uniform piece. The smaller segment of the original broach consists of the medial part of the broach, extending down from the upper face of the osteotomy level. This segment is spring-loaded so that as the surgeon drives the broach into the femur, the medial segment gets caught at some point within the medial cortex. Consequently, as the surgeon advances the body of the instrument further down the femur, the medial segment slides along a track present in the body and remains in a position protruding from the bone. The motion of the segment is constrained by a spring, so that once the rest of the broach has become seated in the bone, the surgeon may drive the medial segment alone into place within the femur, thereby completing the machining operation to form the implantation cavity.